Method of suppressing energy spikes of a partially-coherent beam

ABSTRACT

Accordingly, a method of suppressing energy spikes is provided comprising projecting a laser beam through a mask having a slit pattern comprising a corner region with edges, and a blocking feature with the corner region to reduce energy spikes projected on a substrate. An alternative method is provided, wherein the corner region is modified such that it is replaced by a more tapered shaped region, preferably a triangle. Also provided, are a variety of mask designs incorporating both corner regions, with and without one or more blocking features, and triangular regions, with or without one or more blocking features. The mask designs provide examples of mask modifications that may be used to reduce energy spikes.

BACKGROUND OF THE INVENTION

[0001] A laser beam is used in laser-annealing applications tocrystallize an amorphous material to obtain a crystalline, orpolycrystalline, material. For example, excimer lasers may be used tocrystallize an amorphous silicon (a-Si) film to obtain a polycrystallinesilicon (poly-Si) region. In several of these methods, a mask isinserted into the path of the beam to shape the laser beam before thebeam irradiates the material.

[0002] The mask, used to shape the beam, can in principal have a widevariety of patterns on it. The mask may comprise a patterned layer ofchrome, or other material that blocks the desired wavelengtheffectively, on a quartz substrate, or other suitably transparentmaterial at the wavelength of laser to be used. Common patterns consistof groups of rectangular shapes, including slits and chevrons.

[0003] As the laser beam is projected through these patterns, theintensity profile of the projected beam will be determined by thefeatures that make up the pattern and any optics used to image thepattern on the material. The intensity profile of the laser beam istypically not uniform over the entire pattern. For example, at cornerregions intensity peaks have been noticed. These intensity peaks maycause local damage on the film irradiated by the shaped beam.) One formof damage caused by intensity peaks is agglomeration, which may causethe silicon film to pull away from the region exposed to the highintensity peaks, possibly leaving a void or other non-uniformity.

SUMMARY OF THE INVENTION

[0004] Accordingly, a method of suppressing energy spikes is providedcomprising projecting a laser beam through a mask having a slit patterncomprising a corner region with edges, and a blocking feature with thecorner region to reduce energy spikes projected on a substrate. Analternative method is provided, wherein the corner region is modifiedsuch that it is replaced by a more tapered shaped region, preferably atriangle.

[0005] Also provided, are a variety of mask designs incorporating bothcorner regions, with and without one or more blocking features, andtriangular regions, with or without one or more blocking features. Themask designs provide examples of mask modifications that may be used toreduce energy spikes.

BRIEF DESCRIPTION OF THE DRAWINGS

[0006]FIG. 1 is a plane view showing two prior art mask patterns

[0007]FIG. 2 is a plane view showing a portion of a mask pattern.

[0008]FIG. 3 is a 3D intensity distribution plot corresponding to themask pattern of FIG. 2.

[0009]FIG. 4 is a plane view showing a portion of a mask pattern.

[0010]FIG. 5 is a 3D intensity distribution plot corresponding to themask pattern of FIG. 4.

[0011]FIG. 6 is a plane view showing a portion of a mask pattern.

[0012]FIG. 7 is a 3D intensity distribution plot corresponding to themask pattern of FIG. 6.

[0013]FIG. 8 is a plane view showing a portion of a mask pattern.

[0014]FIG. 9 is a 3D intensity distribution plot corresponding to themask pattern of FIG. 8.

[0015]FIG. 10 is a plane view showing a portion of a mask pattern

[0016]FIG. 11 is a 3D intensity distribution plot corresponding to themask pattern of FIG. 10.

[0017]FIG. 12 is a plane view showing a portion of a mask pattern.

[0018]FIG. 13 is a 3D intensity distribution plot corresponding to themask pattern of FIG. 12.

[0019]FIG. 14 is a plane view showing a portion of a mask pattern.

[0020]FIG. 15 is a 3D intensity distribution plot corresponding to themask pattern of FIG. 14.

[0021]FIG. 16 is a plane view showing a portion of a mask pattern.

[0022]FIG. 17 is a 3D intensity distribution plot corresponding to themask pattern of FIG. 16.

[0023]FIG. 18 is a plot of the critical dimension of blocking featuresrelated to the numerical aperture of the projection system.

[0024]FIG. 19 shows a region following laser annealing using a masksimilar to that shown in FIG. 6.

[0025]FIG. 20 shows a region following a two-pass laser annealingprocess using masks similar to that shown in FIG. 6.

[0026]FIG. 21 is a plane view showing a portion of a mask pattern.

[0027]FIG. 22 shows a region following laser annealing using a masksimilar to that shown in FIG. 21.

[0028]FIG. 23 shows a region following a two-pass laser annealingprocess using masks similar to that shown in FIG. 21.

[0029]FIG. 24 is a plane view of a portion of a mask pattern similar tothat shown in FIG. 21 with the addition of blocking features.

DETAILED DESCRIPTION OF THE INVENTION

[0030]FIG. 1 shows a mask 8 with two examples of common mask patternsused in connection with laser annealing processes. A slit 10 has a firstcorner region 12 and a second corner region 14, as indicated by thedotted ellipses. A chevron 16 also has corner regions 18 and 20, asindicated by the dotted ellipses. It is in the vicinity of cornerregions, that intensity spikes have been notice when a laser beam isprojected through the mask 8 onto a substrate to be laser annealed. Thelaser beam may pass through a variety of projection optics either beforeor after passing through the mask 8.

[0031]FIG. 2 illustrates a portion 30 of a prior art mask pattern foruse in laser annealing processes. The portion 30 includes a slit 32 witha corner region 34. The slit 32 is essentially transparent at thedesired wavelength across its entire area. Transparent refers to an areawith no opaque pattern within the slit, even through there may be slightattenuation from the quartz or other substrate that the pattern has beenformed on. The slit 32 as shown has a width of between approximately 4and 5 micrometers wide. However, the width of the slit is only limitedby the lateral crystal growth length. For example, the slit width couldbe in the range of between approximately 2 and 20 micrometers.

[0032]FIG. 3 is a laser intensity distribution plot for the slit 32shown in FIG. 2. The intensity distribution depends upon the numericalaperture (NA) of any projection system used to image the laser beam thatpasses through the slit 32 onto the material to be crystallized. Theintensity distribution shown in FIG. 3 is based on a projection systemwith a numerical aperture of 0.13. The intensity distribution becomesmore intense in the region at the end of the corner region. An intensitypeak 40 is clearly visible on this intensity plot. The intensity peak 40produces undesirable material uniformity effects. The problemsassociated with the intensity peak 40 are further compounded whenscanning laser annealing processes are used as the corner regions maypass over areas of material that have already been exposed to cornerregions from previous passes. This may contribute to additional damagefrom successive intensity peaks passing over the same region ofmaterial. Elimination of the intensity peak will reduce, or eliminate,damage to the crystallized materials, including agglomeration, and othermaterial non-uniformity effects, caused by the intensity peak.

[0033] Therefore, it would be desirable to have a mask design andcorresponding process that suppresses the corner effect and allows foruniform, crystallized material to be generated upon laser irradiation.

[0034] Several families of designs were considered to modify the cornerregion to reduce, or eliminate, the intensity peak. A first family ofdesigns maintains the outline of the slit, but incorporates beamblocking features in the vicinity of the corner region. A second familyof designs modified the outline of the slit to form a triangular regionin place of the corner region, both with and without beam blockingfeatures. Another family of designs modified the outline of the slit byusing a stepped triangular region in place of the corner region, alongwith the possible addition of blocking features.

[0035] Examples 1 through 7 are based on rectangular slits that areeither four, or four and a half, micrometers wide projected onto amaterial using a projection system with a numerical aperture of 0.13 sothat the resulting intensity distributions can be compared with thosecorresponding to an unmodified slit as described in connection withFIGS. 2 and 3.

[0036] The dimensions provided here are illustrative as the criticaldimensions and spacing of blocking features depend on the projectionsystem to be used.

EXAMPLE 1

[0037] Referring now to FIG. 4, a portion of a mask 50 is shown. Themask 50 has a slit 52 with a blocking feature 54. For example, the slit52 has an end 56 along with a first side 58 and an opposite side 60. Theslit is four micrometers wide measured from the first side 58 to theopposite side 60. The blocking feature 54, as shown, is one micrometerwide, across its narrowest dimension, one micrometer from the end 56,and one micrometer from either side 58 or 60.

[0038]FIG. 5 shows an intensity distribution corresponding to the mask50, shown in FIG. 4. The intensity peak 40 has been shifted away fromthe end of the slit. A secondary peak 42 is also shown. The secondarypeak 42 is well below the baseline intensity, shown at 44. By shiftingthe intensity peak 40 away from the end of the slit, it may be possibleto reduce material damage associated with successive scan exposures,since there is less chance of the intensity peaks overlapping. However,the intensity peak may still damage crystallized material during asingle pass.

EXAMPLE 2

[0039]FIG. 6 shows another example of the mask 50 with the blockingfeature 54 positioned toward the end 56 of a slit 52. As shown the slit52 is 4 micrometers wide. The blocking feature 54 is approximately onehalf micrometer wide and positioned approximately one half micrometerfrom the end 56 and from either side 58 or 60.

[0040]FIG. 7 shows an intensity distribution corresponding to the mask50, shown in FIG. 6. The intensity peak 40 has been shifted slightlyaway from the end of the slit. The intensity peak 40 is significantlyreduced, although still identifiable.

EXAMPLE 3

[0041]FIG. 8 shows another example of the mask 50. In this examplemultiple blocking features 54 are used. As shown, the slit 52 is 4.5micrometers wide. The blocking features 54 are squares with sidesapproximately one half micrometer long. Each blocking feature isseparated from the next by a half micrometer space. There is also a halfmicrometer space from the end of the slit 52 to the blocking features,as well as between either side 58 and 60 and the nearest of the blockingfeatures 54.

[0042]FIG. 9 shows an intensity distribution corresponding to the mask50, as shown in FIG. 8. There is no longer an identifiable intensitypeak. The intensity distribution is essentially flat across the slitregion. The edge of the intensity distribution appears to have shiftedonly slightly from the end of the slit.

EXAMPLE 4

[0043]FIG. 10 provides another example of the mask 50. The corner regionof the slit 52 has been modified. The rectangular corner region has beenreplaced by a triangular end region 64. The triangular end region may bepatterned, as shown, so that the triangular end region corresponds to atriangle having an apex 66 at the position corresponding to the middleof the end of the corner region of the corresponding rectangular slit.As shown the width of the rectangular region is approximately fourmicrometers, which corresponds to the base of the triangular region.

[0044]FIG. 11 shows an intensity distribution corresponding to the mask50, shown in FIG. 10. The intensity peak 40 has essentially the samemagnitude at the peak, as the intensity peak shown in FIG. 3. Theintensity peak 40 is not as wide, however, as it tapers off at an anglealong the edges corresponding to the sides of the triangular region ofthe slit 52.

EXAMPLE 5

[0045] A modified version of Example 4 is provided in FIG. 12. Blockingfeatures 70, 72, and 74 have been added to the triangular end region.The blocking features, as shown, consist of two small triangles 70 and72, which are right triangles with sides that are approximately one halfmicrometer each. Each small triangle 70 and 72 is positioned such thatone of its sides is approximately one half micrometer from the edge ofthe triangular end region of the slit 52. The side may be substantiallyparallel to the slit edge, as shown. A third blocking feature 74 is aright triangle with a hypotenuse that is one micrometer long. The thirdblocking feature 74 has a right angle positioned approximately one halfmicrometer from the apex 66 of the triangular region.

[0046]FIG. 13 shows an intensity distribution corresponding to the mask50, shown in FIG. 12. The intensity peak 40 has been significantlyreduced, although still identifiable. The intensity distribution issufficiently flat for purposes of laser crystallization processes, andshould not induce significant damage.

EXAMPLE 6

[0047] Further modification of Example 5, is shown in FIG. 14. Arectangular blocking feature 76 has been added adjacent to the threetriangular blocking features 70, 72, and 74, such that it isapproximately one half micrometer from each of the triangular blockingfeatures. As shown, the rectangular blocking feature is approximatelyone half micrometer wide and one micrometer in length.

[0048]FIG. 15 shows an intensity distribution corresponding to the mask50, shown in FIG. 14. There are now two intensity peaks 41 and 43identifiable. The intensity distribution is effectively flat forpurposes of laser crystallization processes, and should not inducesignificant damage.

EXAMPLE 7

[0049]FIG. 16 shows another illustration of a modified corner region ofa slit 52 on a mask 50. The corner region has been replaced by a steppedtriangular feature. As shown, the triangular region has a tip region 80,which is one micrometer across at its apex 86, with square steps eachhaving a one half micrometer rise and one half micrometer run. The widthof the rectangular region of the slit 52, as shown, is four micrometerswide.

[0050]FIG. 17 shows an intensity distribution corresponding to the mask50, shown in FIG. 16. The intensity peak 40 has essentially the samemagnitude at the peak as that shown in FIG. 3.. The intensity peak 40 isnot as wide, however, as it tapers off at an angle along the edgescorresponding to the sides of the triangular region of the slit 52. Theintensity distribution appears to correspond very closely with that ofFIG. 11, which corresponds to a triangular region. It is anticipatedthat the addition of blocking features would reduce the intensity peak.Either rectangular, square, triangular or other appropriate shapes couldbe used as blocking features. One or more blocking features could beadded. Based upon the results of shown in FIGS. 7, 9, 13 and 15,blocking features approximately one half micrometer across may beappropriate.

[0051] The above results suggest that the addition of blocking features,in the vicinity of the corners, is one way to obtain a uniform intensityprofile. The critical dimension of the blocking features dependprimarily upon the resolution of the optical system that is used toimage the mask onto the substrate. The resolution depends upon thenumerical aperture (NA) of the optical system. The higher the NA; thehigher the resolution will be (i.e. the optical system will be able toresolve finer features). In the intensity distributions shown above, anumerical aperture of 0.13 was used, corresponding to a resolution ofapproximately 1.6 to 2 micrometers. For this NA, the critical blockingfeature dimension that optimizes the intensity profile is on the orderof 0.35 to 0.7 micrometers. Further optimization may narrow the criticalblocking feature dimension range to between approximately 0.4 and 0.6micrometers, and as discussed above one half micrometer may be optimal.

[0052] As the NA of the optical system changes, the optimum blockingfeature dimension-range will also change. Therefore, we can plot theoptimum block dimension-range as a function of the NA of the imagingsystem. This plot is shown in FIG. 18, which shows the criticaldimension of blocking features in micrometers as a function of thenumerical aperture (NA) of the projection lens. The examples given abovecorresponded to an optical system with an NA of 0.13. The plot shows thecritical dimensions of blocking features for projections lenses havingboth larger and smaller numerical apertures, than the NA of 0.13 used inthe above examples.

[0053] Depending on the NA of the optical system, the critical dimensionfor the size, or width, of blocking features and the spaces betweenfeatures and other portions of the slit will fall within a range.Although for ease of illustration a single value has been used for theboth the blocking feature size and the space between blocking featuresin the above examples, it is possible for the sizes of the blockingfeatures to be different from each other, and that the spaces betweenblocking features be different sizes than the features themselves. Theterm critical dimension corresponds to a range of sizes, and thefeatures and spaces may have varying sizes, preferably the sizes, orwidths, will be within the critical dimension range.

[0054] Referring now to FIG. 19, a polycrystalline region 100 is shownsurrounded by an amorphous region 102. The polycrystalline region 100was formed using a single laser pulse through a mask similar to thatdescribed in connection with FIG. 6. The use of the mask of FIG. 6 witha blocking feature has eliminated agglomeration. However, anotherunexpected crystalline non-uniformity is identifiable. In the end regionidentified at 104, a radial crystal pattern is clearly visible. Thecrystal pattern away from the end region is a more consistent,substantially parallel pattern.

[0055] Referring now to FIG. 20, a polycrystalline region 110 is shownafter completing a multi-pass laser annealing scan using the same maskas that used in connection with FIG. 19. The region 114, whichcorresponds to overlapping end regions, still shows the less desirableradial crystal pattern. Although, a radial crystal pattern is preferableto damage such as that caused by agglomeration, in a preferredembodiment it would be desirable to also reduce, or eliminate, theradial crystal regions.

[0056]FIG. 21 illustrates a mask pattern 116 with a triangular endregion 118. The triangular end region 118 has a base 120 and a length122, that form the basis of the aspect ratio of the triangular endregion. The aspect ratio is the ratio of the length 122 to the base 120.The aspect ratio of the triangular end region is in the range betweenone half and five. One preferred range of aspect ratios is between twoand four. The aspect ratio of the embodiment shown in FIG. 21 isapproximately three.

[0057]FIG. 22 shows a polycrystalline region 200 surrounded by anamorphous region 202. The polycrystalline region 200 was formed using asingle laser pulse through a mask similar to that described inconnection with FIG. 21. In the end region identified at 204, the radialcrystal pattern has been reduced or eliminated. On close inspection ofthe end region identified at 204, a slight fishbone pattern can be seen.This fishbone pattern is closer to the more desirable substantiallyparallel pattern found outside the end region.

[0058] Referring now to FIG. 23, a polycrystalline region 210 is shownafter completing a multi-pass laser annealing scan using the same maskas that used in connection with FIG. 22. The region 214, whichcorresponds to the overlapping end region, no longer shows a radialcrystal pattern. In addition, there is not even a clearly discernablefishbone crystal pattern. The resulting material appears to besubstantially parallel throughout the crystallized region.

[0059]FIG. 24 illustrates a mask pattern 216 which has a triangular endregion similar to that shown in FIG. 21. Additional blocking features218 have been added to provide additional control over beam intensity tofurther reduce, or eliminate, energy spikes. Although FIG. 24, as wellas FIG. 21, are shown as straight-sided triangles, it would also bepossible to use stepped triangles instead.

What is claimed is:
 1. A method of suppressing energy spikes comprisingprojecting a laser beam through a mask having a slit pattern comprisinga corner region with edges, and a blocking feature within the cornerregion to reduce energy spikes projected on a substrate.
 2. The methodof claim 1, wherein the blocking feature is a shape one criticaldimension wide and positioned one critical dimension from the edges. 3.The method of claim 1, wherein the blocking feature comprises multipleshapes one critical dimension wide and positioned one critical dimensionfrom the edges, and one critical dimension from each other.
 4. A methodof suppressing energy spikes comprising projecting a laser beam througha mask having a slit pattern comprising a triangular end region.
 5. Themethod of claim 3, wherein the triangular end region has an aspect ratioof between one half and five.
 6. The method of claim 3, furthercomprising blocking features for reducing energy spikes positionedwithin the triangular end region.
 7. The method of claim 3, wherein thetriangular end region is formed as a stepped triangle.
 8. A mask forsuppressing energy spikes during laser annealing processes comprising:a) a mask pattern having a corner region; and b) a blocking featurepositioned within the corner region.
 9. The mask of claim 8, wherein theblocking feature is one critical dimension wide and positioned onecritical dimension from the edges of the corner region.
 10. The mask ofclaim 9, wherein the critical dimension is between 0.2 and 1.8micrometers.
 11. The mask of claim 9, wherein the critical dimension isbetween 0.2 and 1.0 micrometers.
 12. The mask of claim 9, wherein thecritical dimension is between 0.2 and 1.0 micrometers.
 13. The mask ofclaim 9, wherein the critical dimension is between 0.2 and 0.6micrometers.
 14. The mask of claim 9, wherein the critical dimension isbetween 0.3 and 0.6 micrometers.
 15. The mask of claim 9, wherein thecritical dimension is approximately 0.5 micrometers.
 16. The mask ofclaim 8, wherein the blocking feature comprises multiple small shapes.17. The mask of claim 16, wherein each small shape is one criticaldimension wide, positioned one critical dimension from the edges of thecorner region, and separated from each adjacent blocking feature by onecritical dimension.
 18. A mask for suppressing energy spikes comprisinga mask pattern with a rectangular portion, and a triangular end regionwith edges.
 19. The mask of claim 18, wherein the triangular end regionhas an aspect ratio in the range of between approximately one half andfive.
 20. The mask of claim 18, wherein the triangular end region has anaspect ratio in the range of between approximately two and four.
 21. Themask of claim 18, wherein the triangular end region has an aspect ratioof approximately three.
 22. The mask of claim 18, further comprising ablocking feature positioned within the triangular end region.
 23. Themask of claim 22, wherein the blocking feature comprises multiple shapesone critical dimension wide and positioned one critical dimension formeach other and one critical dimension from any edges of the triangularend region.
 24. The mask of claim 18, wherein the triangular end regioncomprises a stepped triangular region.
 25. The mask of claim 24, whereinthe blocking feature comprises multiple shapes one critical dimensionwide and positioned one critical dimension form each other and onecritical dimension from any edges of the triangular end region.